Solid state electroluminescent device



United States Patent 3,445,724 SOLID STATE ELECTROLUMINESCENT DEVICEJohn J. Lambe, Birmingham, and William C. Vassell, Dearborn, Mich.,assignors to Ford Motor Company, Dearborn, Mich-., a corporation ofDelaware Filed Aug. 31, 1966, Ser. No. 576,372 Int. Cl. H0511 33/10;H01j 63/04 US. Cl. 315-246 ABSTRACT OF THE DISCLOSURE A solid stateelectroluminescent device is formed of a semiconducting crystal ofcadmium fluoride and an ionically conductive solid conductor of betaalumina. The two materials are in intimate contact with one another atan interfacial area and an electric potential is established between thetwo materials.

This invention relates to the art of electroluminescence and moreparticularly to a process and device for the generation of light by thepassage of an electrical current across the interface between asemiconducting crystal and an ionically conductive solid conductor.

The invention has been elucidated by three figures of drawings. Each ofthese rfigures of drawing are strictly schematic and are designed toexplain the theory of operation of the invention.

FIGURE 1 represents schematically an electroluminescent device in whicha single semiconducting crystal is placed in contact with a singleionically conductive solid conductor.

FIGURE 2 is similar to FIGURE 1 except that in this case the singlesemiconducting crystal is sandwiched between a pair of ionicallyconductive solid contuctors.

FIGURE 3 is similar to FIGURE 1 and has been modified to show theprobable mechanism by which this invention operates.

Any suitable semiconductor crystal may be employed in the structuresshown in the drawing, but the preferred substance is cadmium fluoride.The cadmium fluoride is doped with either Samarium, gadolinium, terbium,europium or mixtures of these elements. A preferred structure is cadmiumfluoride doped with 0.1 mole percent europium. It will be noted that thedoping elements are adjacent lanthanides having atomic numbers from 62to 65.

The preferred substance to be employed as the ionically conductive solidconductor is beta alumina. Beta alumina or sodium beta alumina is amaterial conventionally represented by the formula Na O-1lAl O and maybe thought of as a series of layers of A1 0 held apart by a column oflinear Al-O bond chains with sodium ions occupying sites between theaforementioned layers and column. It is widely used in the manufacturingof refractory bricks for lining furnaces subject to corrosion from abasic melt and/ or slag.

According to our present understanding of the electroluminescenceprocess, the light is produced by the action of a strong electric fieldupon a thin surface layer of the electroluminescent, semiconductingmaterial. The origin of this field is, presumably, the charged ions inthe electrolyte which cannot enter the semiconductor and collect at theinterface between the electrolyte and the semiconductor. The externallyapplied electric field drives 7 Claims 3,445,724 Patented May 20, 1969"ice the ions against the interface. A schematic picture of thissituation is shown in FIGURE 3. If the ionic conductor is a liquid,there is no problem of getting the mobile negative ions into intimatecontact with the semiconductor surface and electroluminescence isobserved. However, if the ionic conductor is a solid, there is a realpossibility that the surfaces of the semiconductor and ionic conductorwill be too rough to allow the ions to get close enough to thesemiconductor to generate electroluminescence.

A practical device using a solid ionic conductor was constructed asfollows. A 1 cm. diameter x 0.3 cm. thick disc of cadmium fluoride dopedwith 0.1 mole percent europium was prepared with one face polished on afelt pad impregnated with one micron diamond particles. The electrodecontacting the opposite face was made of an indium-tantalum-mercuryamalgam with a drop of conducting epoxy to connect the electrical leadto the electrode. The ionic conductor was made from a similar sized discof ,B-alumina whose room temperature resistivity was approximately 300ohm cm. One face of this slab was also polished on a felt padimpregnated with one micron diamond particles. The conducting electrodeattached to the 3-alumina was simply the mechanical contact between ametal plate and the surface opposite the polished surface. The polishedsurfaces on the fi-alumina and on the cadmium fluoride were broughttogether and held with a small force and a voltage of approximatelyvolts was applied. Light was emitted from the interface. The light camefrom many spots with a variety of sizes. These were presumably regionsof very good mechanical contact. Wetting the surfaces with a veryconcentrated sodium hydroxide solution before bringing them into contactserved to make an excellent contact and light was observed to comeuniformly from the entire interface.

The collection of interfaces between the metal wires, the semiconductorand the ionic conductor are easily polarized by the application of a DCelectric field. When polarized, the intensity of light emission fallsoff. For this reason the best source of voltage is an AC voltage whichsupplies a backwards voltage and depolarizes the interfaces every cycle.In fact, the best results have been obtained by using a backwardsvoltage approximately three times the value of the forward voltage. Thusthe best results were obtained with a square wave voltage source whichapplied approximately 100 volts for about 50 milliseconds in the forward(light emitting) direction and approximately 300 volts for 50milliseconds in the backwards direction.

We claim as our invention:

1. A solid state electroluminescent device comprising a semiconductingcrystal, an ionically conductive solid conductor of beta alumina inintimate contact with the semiconducting crystal and means forestablishing an electrical potential difference between thesemiconducting crystal and the ionically conductive solid conductor.

2. The structure recited in claim 1 in which the semiconducting crystalis cadmium fluoride.

3. The structure recited in claim 2 in which the cadmium fluoridesemiconducting crystal is doped with samarium, gadolinium, terbium,europium or mixtures of these elements.

4. The structure recited in claim 1 in which the contact between thesemiconducting crystal and the ionically conducting solid conductor ofbeta alumina is enhanced by the addition of a solution of anelectrolyte.

3 4 5. The structure recited in claim 4 in which the elec- ReferencesCited trolyte is a strong solution of sodium hydroxide. UNITED STATESPATENTS 6. The process of generating light comprising passing anelectrical current across an interface between a semi- :12: conductingcrystal of cadmium fluoride and an ionically 5 3271198 9/1966 fg 'gg 'fX conducting solid conductor of beta alumina, the semicon- 3:350:59710/1967 Cole et a1. 313 1O8 ducting crystal and ionically conductingsolid conductor being in intimate contact at an interface to :permit theJAMES W LAWRENCE, P i E i passage of current.

7. The process recited in claim 6 in which the applica- 10 PALMER DEMEOAmstant Exammer' tion of a voltage in the forward direction is followedby CL the application of a high voltage in the backward direction. 31310

